February 28, 2006 A DECOMPOSITION OF LIFE EXPECTANCY LEVELS AND TRENDS John Bongaarts Population Council New York Correspondence: John Bongaarts Population Council 1 Dag Hammarskjold Plaza New York, NY, 10017 [email protected] Abstract: Since 1800 life expectancy at birth has doubled from about 40 to near 80 years in high income countries. Pessimists expect these improvements to end soon because we are approaching biological limits, but optimists predict continued rapid improvements with no limits. To shed light on this controversy, past trends in the juvenile, background and senescent components of life expectancy are examined in 16 high income countries. Large increases in conventional life expectancy before 1950 are found to be primarily attributable to reductions in juvenile and background mortality. After 1950 the rate of improvement in life expectancy slowed because declines in juvenile and background mortality slowed, but senescent mortality fell more rapidly than before, thus becoming the main cause of rising life expectancy at birth. The role of smoking in the past half century is also quantified. In the future, background mortality and juvenile mortality will have little or no impact on longevity because they have reached very low levels. There is however no evidence of approaching limits and life expectancy will likely improve at a rate of approximately 1.5 years per decade due to continued declines in senescent mortality. 2 One of the most notable achievements of modern societies is a large rise in human longevity. Since 1800 life expectancy at birth has doubled from about 40 to near 80 years. The causes of this massive decline in mortality include improvements in standards of living, nutrition and education, the implementation of wide-ranging public health measures and more effective and accessible medical care (Riley, 2001). Recent mortality trends are well established, but there is considerable disagreement among demographers and biologists about what lies ahead. Pessimists believe we are approaching limits to life expectancy while optimists expect continued rapid improvements with no limits. Much is at stake: improvements in longevity are a key cause of skyrocketing costs of pensions and healthcare for the elderly. After a brief review of the controversy about future trends, this study examines past trends in the components of life expectancy at birth. The projection of these components can provide the basis for assessing plausible future trends in longevity. The focus throughout will be on high-income countries with low levels of mortality. Background: From pessimism to optimism Reliable historical estimates of mortality are available for a small number of countries. Figure 1a plots past estimates of life expectancy at birth for females in 16 high-income countries: Austria, Belgium, Canada, Denmark, England and Wales, Finland, France, Italy, Japan, Netherlands, New Zealand, Norway, Spain, Sweden, Switzerland, USA (Human mortality database, 2005). 1 The longest plotted time series start in 1850 and end in 2000 and the shortest are from 1950 to 2000. Trends for male life expectancy at birth in Figure 1b are broadly similar, although males live, on average, a few years less than females. In all sixteen countries large increases in life expectancy occurred. The only significant interruptions in this overall upward trend are attributable to the global influenza epidemic in 1918-1919 and two World Wars. Differences between these countries have narrowed considerably over time, leaving most countries today with life expectancies close to the average for the group, regardless of historical patterns. In recent decades, countries have moved in tandem within a narrow range to reach an average in 2000 of 81.5 years for females and 75.8 for males. This remarkable convergence is presumably due to a reduction in disparities among countries in standards 3 of living, nutrition and public health measures, and increasingly rapid diffusion of advances in medical treatment, drug therapy and life style among countries. Another notable development evident in Figure 1 is that the pace of improvement in female life expectancy is less rapid in recent decades than in the century before the 1950s and male life expectancy stalled in the 1950s and 1960s. The future implications of these recent trends are the subject of a contentious debate. Pessimists believe that future life expectancy has an upper limit of about 85 years and they provide biological and demographic evidence in support of this view (Fries 1980; Olshansky et. al. 1990). The biological argument considers mortality after the reproductive ages to be beyond the reach of Darwinian forces of natural selection. As a result, an “intrinsic” biologically determined age pattern of senescent mortality is said to exist which rises steeply with age after about age 30 in humans. This pattern is “expected to remain invariant unless the genome itself is modified” (p.252, Carnes et al. 1996). The pessimists’ demographic argument claims that improvements in life expectancy at birth can only result from declines in “premature” mortality among children and young adults. In contrast, senescent mortality at older ages is considered largely immutable, because at the end of the natural lifespan “everything comes apart at once and repair is impossible” (p.135, Fries, 1980). Up to the 1980s this pessimistic perspective was accepted by many demographers. For example, Bourgeois-Pichat (1978) proposed “biological” limits for life expectancy of 80.3 years for females and 73.8 years for males. Population projections prepared by the United Nations at various times from the 1950s to the 1980s included a maximum. This pessimism led to the consistent underestimation of future improvements in longevity in projections made before the 1990s (Keilman, 1997). Several developments during the 1990s led to the demise of this pessimism. First, many proposed past limits to life expectancy were broken, often soon after they were introduced (Oeppen and Vaupel, 2002). This continuous need for an upward revision of supposedly fixed limits is most clearly evident in the past record of the UN projections, which raised their maximum life expectancy several times before the mid-1980s. The more recent UN projections have abandoned the practice of imposing limits. Second, age specific death rates at the oldest ages show no evidence of leveling off. On the contrary, 4 these rates exhibit steady declines (Lee and Carter, 1992; Kannisto et al. 1994). Third, the pace of improvement in old age mortality shows no relationship to the level of old age mortality (Wilmoth, 1997). If limits exist, one would expect countries close to the limit to experience smaller and slower improvements than countries that are farther from the limit. Finally, the claim that mortality at the oldest ages is not subject to the forces of natural selection is being questioned (Lee, 2003). Moreover, it is doubtful that a biological argument can contribute insight into the potential future impact of medical interventions, even if it explains the exponential rise with age in post reproductive age mortality. In their recent writing, the pessimists seem to have made a concession. Instead of being immutable, the limits to life span are now referred to as “a mortality schedule, that in the absence of medical improvements cannot further be reduced” (italics added, p.505, Olshanski, et. al. 2002). Any such improvements “manufacture survival time by saving the lives of people who would otherwise die” (p. 505). This language represents a significant change in position, or at least in the position as interpreted by many readers of the earlier work of Olshansky and his colleagues. It is quite possible, indeed likely, that most improvements in mortality at older ages in the future will be “manufactured” through medical interventions, but if that is the case the net result will still be a life expectancy beyond the proposed limits of about 85 years. As noted by Wilmoth (2001), the difference of opinion between Olshansky and Carnes, and other demographers about likely future trends in life expectancy now appears to be smaller than is widely presumed. A recent panel report of the National Research Council concludes that if any limits exist, they are far above current levels, and that projections therefore should not impose ceilings (National Research Council, 2000). This view is now widely held in the demographic community. There is, however, no agreement about most plausible future trends. Optimists such as Oeppen and Vaupel (2002) observe that best-practice life expectancy has increased by 2.5 years per decade for the past century and a half and they conclude that a “reasonable scenario” would be for this trend to continue in future decades. Manton et al. (1991) also anticipate much higher levels of life expectancy in the future, due to the development of interventions to address chronic disease at advanced ages. This view is not shared by most national and international agencies that are 5 responsible for preparing official country projections. Their projections are considerably more conservative even if they do not impose limits. For example, projections for the next half century made by the UN in 2004 (without a life span limit), expect female life expectancy in the US to increase at a rate of 1.1 years per decade (United Nations, 2005). Similarly, the US Social Security Administration assumes a rise in life expectancy of only 0.8 years per decade over the same period (Board of Trustees OASDI, 2005). These projected rates of future improvement are less than half the rate considered reasonable by Oeppen and Vaupel (2002). The question of what lies ahead is still unsettled. Components of life expectancy levels and trends The disagreement about future trends is in part attributable to different interpretations of past trends. To make progress it is therefore necessary to arrive at a better understanding of the past. To this end a procedure for decomposing life expectancy is proposed.